Yuya OnoUniversity of Electro-Communications, Graduate School of Informatics and Engineering, Tokyo Japan

Yuichi IdaUniversity of Electro-Communications, Graduate School of Informatics and Engineering, and Research Station on Seismo Electromagnetics, Tokyo Japan

Yasushi KasaharaUniversity of Electro-Communications, Graduate School of Informatics and Engineering, and Research Station on Seismo Electromagnetics, Tokyo Japan

Yasuhide HobaraUniversity of Electro-Communications: Graduate School of Informatics and Engineering; Research Station on Seismo Electromagnetics; Center for Space Science and Radio Engineering, Tokyo Japan

Abstract

The presence of ionospheric perturbations in possible association with two huge earthquakes (Noto-hanto peninsula and Niigata-chuetu-oki earthquakes) in 2007 was studied on the basis of a conventional statistical study for a particular propagation path from the JJI transmitter in Miyazaki, Kyushu, to Moshiri in Hokkaido. This is based on automatic routine-based signal processing, in which the trend as the average nighttime amplitude is significantly decreased, with almost simultaneous significant enhancement in the night-time fluctuation as the night-time integration of negative fluctuation from the average. It is, however, shown that this routine-based signal analysis sometime suffers from artificial (or man-made) effects. Thus, in this study, we propose an additional use of principal component analysis (PCA) for simultaneous observation of a few VLF/LF propagation paths. With the application of this PCA method to multi-path data, the artificial effects can be reasonably removed, and also only the geophysical effects associated with earthquakes are detected, by focusing mainly on the third principal component. The satisfactory separation of the principal components is made possible by pre-analysis of the VLF data (extraction from the raw data of the average over a whole year). This PCA method enables us to identify the seismogenic effects in association with earthquakes with smaller magnitudes, down to M 5.5 or M 5.0.